Turbine case



July 7, 1931.. r A v, RP V 1,813,404

TURBINE CASE Filed Aug. 15', 1928 2 Sheets-Sheet 1 I ANEjN/TOR' M M ATTORNEY A. v. KARPOV July 7, i931.

TURBINE (EASE 2 Sheets-Sheet 2 Filed Aug. 15 1928 INVENTOR a f? m ATTORNFTY Patented July 7, 1931 UNITED STATES PATENT OFFICE TURBINE CASE Application filed August 15, 1928. Serial No. 299,711.

My invention relates to casings for turbines, etc., including the scroll casings of hydraulic turbines.

Ordinarily, these casings are of arbitrary form in cross section, and are subject to stresses in two general directionsone stress acting against the circumference of the cross section, and the other stress being along the genatrices of a spiral or scroll. The distribution of stresses in these directions is quite uncertain or non-uniform, and heavy bending moments may occur at various points in the scroll case, and particularly at points where the scroll case is connected to the speed or retaining ring, with consequent deflections.

One object of my invention is to make the crosssectional shape of the scroll case of such form that the circumferential bending moments are avoided.

Another object of my invention is to so.

form scroll cases that a saving in material is effected without weakening of the cases.

As illustrative of the following description, I have prepared the accompanying drawings, wherein Figs. 1 and 2 are sectional and plan views, respectively, diagrammatically showing one change in shape that tends to occur in turbine casings having an initial contour of a given form; Figs. 3 and 4 are cross sectional and plan views, respectively, showing a still further change in shape WhlCh may occur by reason of internal pressures; Figs. 5 and 6 are sectional and plan vlews, respectively, of a circular casing of such cross sectional form that bending m0- ments are avoided; Figs. 7 and 8 are sectional and plan views of a modification of the caslng of Figs. 5 and 6; and Figs. 9 and 10 show plan and sectional views, respectively, of a scroll casing.

The problem of determining stresses in a scroll case of a hydraulic turbine is a rather complicated one on account of the peculiar unsymmetrical shape of the scroll case and the way in which it is stiffened by the speed ring.

A clearer understanding of the conditions can be gained by starting out with the study of simpler shapes leading toward the more complicated shape of a scroll case.

Referring now more particularly to the form of casing shown in Figs. 1 and 2, at 11 I have indicated the initial form of a casing. If water under pressure is introduced into this casing deformation tendencies are developed, these deformation tendencies being in an amount equal to pressure times the casing area. Assuming that the casing be made of thin sheet metal, changes from its cross-sectional form as indicated by the oval lines, through the direct pressure or expansive tendency of the water are small, and there will be no bending moments developed, if the casing is circular. By the term bending moments, I means those deformations which tend to occur in a shell or casing of the scroll type, as shown in Fig. 9, for instance, which result through stresses or pressures created in such unsymmetrical casings, and not present in circular casings, as shown in Figs. 1 to 8. As above stated, such deformations which tend to occur in the ring 11 are those resulting only from the tension or compression within the shell, acting against the circumference of the ring. The stresses upon any circumferential section of the ring is equal to the area of such section multiplied by the water pressure.

Considering now the deformations, forces, bending moments and stresses acting along the circumference of the ring section, it is to be noted that they depend on the shape of the ring section. A ring as shown in Fig. 1 will develop bending moments that will tend to change the shape of the ring as indicated by dotted lines. This change in shape will tend to increase the length of the outside and decrease the length of the inside circumference of the ring and consequently increase the stresses in the outer and decrease the stresses in the inner circumferences-of the ring.

A ring 12 shown in Figs. 3 and 4 will develop bending moments that will tend to change the shape of the ring as indicated by dotted lines and consequently decrease the stresses in the outer circumferences and increase the stresses in the inner circumferences of the ring. These changes in shape of the ring section will be partly counteracted by the restraint along the circumference of the rmg.

A certain shape of a ring section exists that does not develop any bending moments acting along the circumference of the ring section. Such ring sections will be referred to as a fundamental ring. If a straight piece of thin pipe, subjected to uniform inside water pressure, would be considered, then the circu lar cross section would be the shape of the fundamental section of such pipe. This fundamental section does not depend on the water pressure. If such piece of pipe is bent to a hollow shell ring, then the fundamental section changes from a circle to a different shape and the deviation from the circular shape is increased with the decrease of the ratio of the main center line radius of the ring to the radius of the pipe. As in a straight pipe, the fundamental shape of a hollow shell ring does not depend on the water pressure and so long as the shell is thin, it depends only on the relation of the size of the ring section to the size of the ring.

In order to find the fundamental section, a small element of the ring out out by two planes a?) and (20 going through the main axis, as indicated in plan-in Fig. 6, is to be considered. For such an element the moments and tensions along the circumference of the ring section can be figured and a ring section with no bending moments can be found. Figure 5 shows approximately a ring section formed in this way. A fundamental ring has no bending moments and no deflections due to bending moments; the stresses along the circumference of the ring section are the largest at the inner part and decreasing toward the outer part of the ring.

Changing the ring section from a fundamental to a difierent shape will induce, in the ring section, circumferential bending moments and deformations due to those moments. These deformations will tend to bring the ring back to its fundamental shape. The change in the ring section will also redistribute the stresses along the ring section, increasing the stresses in the outer part and decreasing them in the inner part of the ring.

The next shape to be considered is the ring 13 shown in Figs. 7 and 8. This shape is formed by cutting away a part of the ring section of a hollow shell ring and by fixing the ends of the shell. To the previously made references and assumptions, the following assumptions will be added:

The ends of the shell are to be fixed in such way that they extend tangential to the original ring section and the restraint of the fixed ends is such that it does not interfere with the deflections along the main circles.

If such a ring is formed by cutting away of a part of a section of a fundamental ring, it will be referred to as a fundamental ring with fixed shell ends. Such ring has all the characteristics of a fundamental ring, so far as the remaining part of the shell is concerned, including the absence of bending moments in the ring section and at the points where the ends of the shell are fixed. If the ring section of such ring is changed from fundamental to some different shape, bending moments and deformations due to these bending moments along the circumference of the ring section will be induced.

The additional stresses imposed on the ring by these bending moments will be partly relieved by the deformation of the ring, except atthe points where the ends of the shell are fizelzdand where no deformations are poss1 e.

Assuming that the ends of the shell are fixed by connecting them to a single, circular, central ring 14, the forces acting upon such central ring can be determined. These forces will be uniform radial forces, acting at the outer circumference of the center ring. For the present it is sufficient to mention that such forces do not develop any bending moments in a circular ring.

Considering finally the scroll case of Figs. 9 and 10, it is to be noted that so far as the cross-section of the scroll case is concerned, a hollow shell ring with fixed shell ends approaches very closely the conditions of a scroll case.

So far as the plan view is concerned, a scroll case differs vastly from such ring.

The principle of a fundamental ring can be applied also to a scroll case and a fundamental scroll case formed. The elements (for instance sections such as that defined by the lines (Z6 and (If) of such scroll case have to be formed by vertical planes radial to the curve going through the centers of the scroll case sections. The main difference between a fundamental ring with fixed shell ends and a fundamental scroll case will be that for a fundamental ring the shape and dimensions of the ring sections are the same at every point of the ring, whereas for a fundamental scroll case, the shape and dimensions of the scroll case sections will change at every point. Taking a single, sufliciently small element of a scroll case, it will be always possible to find a corresponding ring that is composed of identical elements. 7 i,

The ends of the scroll case shell are connected to a speed ring and radial forces are transmitted from scroll case to the speed ring.

These forces are varying along the circumference of the speed ring and result in appreciable bending moments acting along the circumference of the speed ring.

These moments tend to change the shape of the speed ring from a circular to some irregular shape.

For the proper functioning of the turbine, the speed ring is to be stiff enough to withstand these bending moments without changing its shape.

Making this assumption, the limits for the stresses along the circumference of the scroll casecan be indicated.

For a ring, the restraint along the main circles was a positive and certain one and the conditions of this restraint did determine the stresses along the circumference of the rlng. V

For a scroll case, the conditions of the restraint along the circumference of the scroll case are uncertain, but it can be reasonably assumed that they are less severe than in a ring.

Under this assumption, the stresses in the different elements of the scroll case can be assumed to be very nearly the same as the stresses in rings corresponding to these elements.

The difierence in restraint may result in lowering of the stresses along the circumference of the scroll case and in changing of the stresses along the scroll case sections, but for a fundamental scroll case these difference in stresses are very slight.

Any shape of a scroll case section can be formed by changing the shape of a fundamental scroll case. Increasing the deviation from fundamental shape will increase the bending moments along the section of the scroll case, the deformations, and the influence of the restraint along the scroll case on the stresses.

As in the case of the ring, the stresses in the shell will be considerably reduced by the deformation, but at the points where the shell is connected to the speed ring, the whole amount of bending moment is to be carried.

In so shaping a scroll case as in Figs. 9 and 10, as to eliminate bending moments and deformations, the stresses upon a given section or element of a scroll case as defined for example by the lines ab and ac of Fig. 6, are ascertained, and an element or section asdedf of Fig. 9 is so formed as to effect a substantially uniform or average distribution of the stresses over the area of such element de df.

If such elements are not selected of a too great circumferential area, the non-uniformity of pressures against the wall surface of a given element is substantially negligible in practice. The degree of elimination of bending moments or of accuracy is dependent upon the size of each area or element for which equivalents are made in a given scroll case.

As stated hereto-fore, the shaping of a ring section so that it will constitute a fundamental ring section and therefore one wherein there are no bending moments under fluid pressure, will be determined by the relation of the radial diameter of such ring section to the radius of the curvature of the ring. Given these two dimensions, the form which such a ring section must have in order to constitute it a fundamentalring can be determined by a mathematician.

Referring now more particularly to Fig. 9, the lines de-df define a segment of the scroll case. The designer of the case will take the average radial dimension of the scroll case lying between these two lines and the radial dimension of the curvature of this segment from the center d and utilizing these two known quantities will, through the process of differential equation, determine the fundamental ring section therefrom. This segment would then be shaped in accordance with such ring section, there by assuring that the segment would not be subject to bending moments. The remainder of the scroll case willbe similarly divided into segments and shaped to eliminate bending moments therein. In a scroll case so formed, the circumferential tension stresses at all points circumferentially of any given segment will be constant. By circumferential is here meantin a direction circumferentially of the segments as viewed in Fig.

10, rather than circumferentially of the scroll case as a whole.

Summarizing, the following conclusions can be reached:

1. To eliminate the bending moments and deformations, the cross section of the scroll case is to be given the fundamental shape.

2. If the scroll case is shaped differently, then deformations will result and bending moments will occur that will be particularly heavy at the connection between scroll case and speed ring. The stresses in the scroll case will depend on the restraint along the circumference of the scroll case.

3. The stresses in a scroll case can be safely figured within close limits only for the fundamental scroll case.

4. If a steel plate scroll case is to be encased in concrete, then only the fundamental scroll case, having no bending moments and only very small deformations, can be reasonably expected to divide the load between the scroll case and concrete. A scroll case of different shape will throw the wholeload on the concrete until given a chance to deform sufficiently to take the load.

I claim as my invention:

A turbine case of scroll form, composed of unitarily-connected segmental portions each of which constitutes approximately a fundamental ring section, based on the average radial dimension of that particular section.

In testimony whereof I the said ALEXAN- DER'V. KARPOV have hereunto set In hand.

ALEXANDER V. KA POV. 

